@@ -14,10 +14,12 @@
  * file.
  */
 
 #import "OFEnumerator.h"
 
+OF_ASSUME_NONNULL_BEGIN
+
 /*!
  * @protocol OFCollection OFCollection.h ObjFW/OFCollection.h
  *
  * @brief A protocol with methods common for all collections.
  */
@@ -38,7 +40,9 @@
  *	  specified object.
  *
  * @param object The object which is checked for being in the collection
  * @return A boolean whether the collection contains the specified object
  */
-- (bool)containsObject: (id)object;
+- (bool)containsObject: (nullable id)object;
 @end
+
+OF_ASSUME_NONNULL_END